The present invention relates generally to continuous weight loss systems and more specifically to a new, improved control for continuous weight loss systems.
In the field of continuous weight loss systems, the devices include a batch bin located above a continuous loss bin. A feeding device at the outlet of the continuous loss bin is controlled to produce a desired feeding rate. The feeding device is driven by an error signal which is a function of desired weight loss against actual weight loss in the continuous loss bin. Prior systems have used analog means to provide the desired weight signal and compare it on an analog basis with the actual weight signal to produce the error signal. Generally, measurement of the weight in the continuous loss bin is used and compared against an analog derived signal whose initial weight at transfer of the material from the batch bin to the continuous weight loss bin is fixed to a predetermined value. Similarly, the analog systems had to be zeroed during the weight transfer or driven to the predetermined amount. By using a predetermined amount in the analog system, the desired weight does not take into account the actual weight of contents of the continuous loss bin and any variation in the batch amount. Thus the actual weight in the weight loss bin compared against a predetermined and therefore inaccurate desired weight produces erroneous control of the feeding device. Similarly, analog systems use electronic components whose values change with age, thus introducing additional error into the system.
Patents which are considered the state of the art having analog systems are as follows:
U.S. Pat. Nos. 3,319,828; 3,329,313; 3,529,139; 3,532,253; 3,828,869; 3,889,848.
In loss-in-weight, and weight-differential systems presently available, the primary control signal--that is the pre-set weight per unit time signal--is established on an analog basis by a device such as a ramp generator using a capacitance component. Since the accuracy of this capacitance signal deteriorates over a relatively short time, it becomes necessary to re-establish the weight per unit time signal, by refilling the continuous loss bin after a short time interval.
One of the major disadvantages of this concept is that during each refill cycle, the feeding device is locked onto a feed rate signal which is related to the previously established primary rate signal. This results in the feeding device running at some feed rate value not actually related to the required rate during that refill cycle time, therefore causing the feeder to be out of control every time the refill occurs.
The requirement for frequent refilling makes it difficult to provide low capacity continuous weigh feeding; and the out of control cycling time makes it difficult to provide accurate inventory control, since the amount of material fed out during the re-fill time is lost.
An attempt to provide a continuous control of the correct weight of flow is shown in U.S. Pat. No. 3,319,828 by using a load cell to determine the weight in the lower or continuous bin during normal flow and a load cell to measure the combined weight of the two bins during transfer of the materials from the batch bin to the continuous loss bin. Mechanical switching between the two load cells is provided. By using a single load cell to determine the weight of both bins, the cell must have twice the capacity. Thus, with a 0.1 percent accuracy, the error in the combined weight measurements is unacceptable. Similarly, by using a single load cell for continuous measurement during the transfer of material, the measurements would be unstable because of the influence of the material moving from the batch bin to the continuous loss bin.
Thus there exists a need for a control system for a continuous weight loss system wherein the weight of the continuous loss bin and the batch bin are continuously monitored and used to continuously control the feed mechanism without the inherent problems of the analog systems of the prior art.